This invention relates to olefin polymerization catalysts and more particularly relates to unsupported titanium halide based, ester-modified catalysts useful for polymerization of olefins such as propylene.
Olefin polymerization catalysts based on transition metal compounds and aluminum alkyls are well known in the art. The use of titanium trihalide compounds as a polymerization catalyst component to produce crystalline alphaolefin polymers has been reported widely. One group of titanium trihalide based catalyst components generally are referred to as "unsupported" catalysts. Typically, these are produced by reducing a titanium tetrahalide, such as titanium tetrachloride, with an agent such as an aluminum alkyl and forming a complex with the resulting titanium trihalide with an electron donor compound together with treatment by a Lewis acid, such as titanium tetrahalide. A widely-published unsupported catalyst system is based on a complex of titanium trihalide with an ether, thioether or thiol.
U.S. Pat. No. 4,210,738 describes a titanium halide-based catalyst in the delta crystalline form having a color tending toward violet made by reducing titanium tetrachloride with an aluminum alkyl, treating the reduced solid with an ether, thioether or thiol complexing agent and reacting with additional titanium tetrachloride.
U.S. Pat. No. 4,305,920 describes a titanium trihalide catalyst complex containing an ether which is precipitated in the presence of an ester having at least 11 carbon atoms.
U.S. Pat. No. 3,984,350 describes a highly catalytically active titanium trihalide composition in the beta crystalline form having a brown color formed by reducing titanium tetrachloride with an aluminum alkyl and reacting, under controlled conditions, the resulting reduced product with a complexing agent such as an ether and additional titanium tetrachloride.
U.S. Pat. No. 4,295,991 describes a method to treat an unsupported catalyst with olefin monomer such that the resulting catalyst particle is non-friable.
U.S. Pat. No. 4,400,494 describes use of an unsupported catalyst with an aluminum alkyl halide compound containing a specific halogen/aluminum atomic ratio.
U.S. Pat. No. 4,324,693 describes an unsupported catalyst in which the molar ratio of complexing agent to titanium compound is greater than 1.15.
U.S. Pat. No. 4,422,956 describes activation of a complexed reduced titanium halide composition with a Lewis base complexing agent and a haloalkylchlorosilane.
U.S. Pat. No. 4,190,550 describes reduction of titanium tetrachloride by aluminum metal in benzene in the presence of aluminum chloride. The resulting liquid phase is treated with butyl acetate and a solid is precipitated by addition of heptane which is activated with titanium tetrachloride.
Other patents relating to ether-complexed, reduced titanium trihalide catalysts include U.S. Pat. Nos. 4,039,521, 4,240,928, 4,251,388 and 4,315,090.
Although some references generally teach Lewis base complexing agents, which would include compounds other than ethers, thioethers or thiols, the art also teaches that Lewis bases such as esters do not form a very active polymerization catalyst in comparison to an ether-complexed catalyst component. For example, in the prosecution history of U.S. Pat. No. 4,210,738, the applicant withdrew a paragraph suggesting that esters are suitable complexing agents based on experimental findings that esters such as ethyl acetate were inoperative.
Applicants have found, contrary to indications in the art, that highly active olefin polymerization catalyst components can be formed from aluminum alkyl-reduced titanium halides complexed with a special class of ester compounds.